Progress in drilling techniques in recent years has been remarkable, and the developments of improved techniques for controlling drilling fluids applied to subterranean formations has permitted maintaining a high specific gravity under high temperature conditions. Thus, it becomes possible to drill deeper rock formations smoothly.
After completion of an well, oil production can be carried out, but formation damages as described below often occur. More particularly, if drilling mud and solution filtrated from a cement slurry penetrate into reservoirs which contain clay, the clay is swollen and pluggs pores to result in a lowering of permeability; or, when reservoirs of sandstone have not been fully concreted, sand flows into the well with the oil and gas, thus decreasing the productive capacity. As countermeasures to resolve such damages, various workovers can be carried out. In these cases the use of a clear brine as completion and workover fluids, without solids such as bentonite and barite, have been recommended. That is, the clear brines heretofore in use typically contain soluble salts used in an amount sufficient to obtain a desired specific gravity, hydroxyethyl cellulose as viscosifier, which is not substantially affected by the soluble salts, and if desired, various additional materials.
However, the temperature in the well tends to rise as the well becomes deeper, and a lowering of rheological properties of the completion and workover fluids results in the problem that the completion and workover fluids can not fully display their capacity for cleaning the sand or other deposits in the well up to the ground surface. Thus, it has been found that the necessary working time becomes extended and the resulting sand or other deposits can not be completely cleaned out.
Accordingly, only when a method that is able to prevent a lowering of rheological properties of completion and workover fluids heretofore in use can be found, it become possible to fully enjoy the beneficial properties of such fluids.